Currently, with the rapid development of video services, subscribers have an increasingly high demand for the bandwidth. Although the 2 Mbps bandwidth of a digital subscriber line (DSL) is sufficient for the current data transmission services, it is difficult to satisfy the video services. A new broadband access network, such as an Ethernet Passive Optical Network (EPON) and a Gigabit Passive Optical Network (GPON), further promotes the bandwidth of the access network. However, because the time division multiplexing and burst mode technologies are employed, the cost of the system is rather high. Moreover, the bandwidth for a single subscriber merely increases slightly, because all subscribers share the bandwidth. With the development of the video services, especially the demand for high definition video services, an access network with a higher bandwidth is needed. A wavelength division multiplexer passive optical network (WDM-PON) not only inherits the bandwidth characteristic of a WDM network but also has the low cost characteristic of a passive optical network (PON), which thus is focused by many companies, standard organizations, and research institutions. Prototypes of the WDM-PON have been developed in many companies and the formulation of standards has been put on the agenda. However, on the whole, the WDM-PON is not very mature and needs to be improved in many aspects, for example, colorless light source, temperature compensation, network upgrade, and so on. The colorless light source is mainly employed to reduce the maintenance cost of the network and simplify the network management. Nowadays, the colorless light source mainly includes an injection-locked Fabry Perot laser diode (FP-LD), a reflecting semiconductor optical amplifier (RSOA), and a superluminescent light emitting diode (SLED). Among the three light sources, the injection-locked FP-LD has the lowest cost and is most likely to be popularized. However, the FP-LD has multiple longitudinal modes. If the wavelength of the incident light cannot be aligned with one of the longitudinal modes, a threshold of the incident light is increased, and an output optical power of the FP-LD is significantly lower than that achieved at the moment of the alignment. Moreover, the mode varies with the changing of the external temperature, so that the output optical power is changed to a great extent.
FIG. 1 is an injection-locked FP-LD technical solution in the prior art. A broad-band light emitted from a broad-band light source 10 enters a wavelength division demultiplexer (DMUX) 12 via a circulator 11. The spectrum of the broad-band light is divided by the DMUX 12 into many narrow-band lights. The narrow-band light with a different wavelength is output from each channel of the DMUX 12 and incident upon a corresponding FP-LD laser 13. The FP-LD 13 outputs a light with the same wavelength as that of the incident light and suppresses lights with the other wavelengths. A signal may be loaded into the output light of the FP-LD 13 by modulating a driving current of the FP-LD 13. The lights of all FP-LDs 13 are combined into a multi-channel WDM signal via a wavelength division multiplexer (MUX, the same element as the DMUX) and then output via the circulator 11.
As shown in FIG. 2, a structure of a common FP-LD laser assembly in the prior art includes an FP-LD chip 21, a monitor photo detector (MPD) 22, and an amplifier 23 (optional). To ensure a constant output power, the reflection at a rear end surface of the FP-LD chip is not ideal total reflection but has a loss, so that a part of laser energy sent by the FP-LD chip is incident upon the MPD located behind the end surface via the end surface. In fact, the MPD is a photodiode capable of converting the incident laser into a current and outputting the current, and then the current is amplified by the amplifier to serve as a feedback input to a laser driver chip, thereby ensuring that the FP-LD laser outputs a constant optical power. In addition, the changing of temperature influences the output power and wavelength of the laser greatly. In order to further ensure the stable power and wavelength, the laser assembly usually further includes a thermoelectric cooler (TEC) and a thermistor 24 (of negative temperature coefficient). A temperature-controlling current is adjusted according to a temperature of the laser diode (LD) measured by the thermistor to realize a closed loop negative feedback, so that the LD maintains a constant temperature, thereby ensuring the stable power and wavelength.
The FP-LD is a multi-longitudinal mode laser and usually has a low side mode suppression ratio (SMSR) (a ratio of a power of a main mode to that of a neighbor mode). When the wavelength of the incident light corresponds to a center of the main mode and when it corresponds to a valley between the modes, the output optical powers are significantly different from each other. Due to a drift of an arrayed wavelength grating (AWG) channel and FP-LD temperature, as well as the inconsistency between the FP-LD longitudinal modes, the output power of the FP-LD is random, and when the incident light corresponds to the valley of the longitudinal mode, a threshold of the injection locking is increased and the output power of the FP-LD is lower. In order to solve the problems caused by the mode misalignment, one common solution is to coat an anti-reflection film on a front end surface of the FP-LD to weaken the modes of the FP-LD, which nevertheless introduces a large power penalty. In the technical solution of the prior art, if the reflectivity of the front end surface is reduced to 1%, the modes of the FP-LD are weakened. However, the mismatch between the modes of −0.2 nm and +0.07 nm results in a power penalty of 3 dB. In addition, during the actual implementation, the inventors found that, if a specific wavelength calibration is not performed on each FP-LD, the mismatch of the modes may occur to a great extent, which results in a difference between optical network units (ONUs), thereby further affecting the reliable operation of the system. On the other aspect, if the wavelength calibration is performed on each FP-LD, the element cost and maintenance cost are greatly increased. If the AWG is not an athermal AWG of a high cost, the common FP-LD wavelength control process is no longer feasible, due to the drift of the AWG channel.